1. Field of the Invention
The present invention relates to a carbazole compound and a new organic electroluminescent device. In particular, the present invention relates to a 3,6-diphenylcarbazole compound which realizes an organic electroluminescent device having high durability and high luminous efficiency and an organic electroluminescent device including the same. Further, the 3,6-diphenylcarbazole compound of the present invention can be applied to other fields such as photoelectric conversion devices and charge transporting materials for organic photosensitive elements.
2. Discussion of the Background
Organic thin film electroluminescent devices have been noted because of having the following advantages:    (1) viewing angle is wide, and visibility is high because the devices are a self-luminescent device; and    (2) space can be saved because the devices are a thin film type perfectly solid device.
Therefore, in recent years, research on commercialization thereof has been conducted. However, there are the following problems to be overcome at the present time:    (1) the energy conversion efficiency and the luminescent quantum efficiency have been further improved; and    (2) the stability of the organic thin film with time (i.e., durability of the device) has to be further improved.
Organic electroluminescent devices through the use of low-molecular materials and through the use of polymers have been reported up to now. With respect to the low-molecular material systems, -realization of increases in efficiency due to adoption of various laminated structures and an improvement of the durability by appropriately controlling a doping method have been reported. However, in the case of an aggregate of low molecular materials, it is reported that change in the state of film occurs with a long time and, therefore, there is an essential problem in the stability of the film.
In contrast, with respect to polymer based materials, researches have been intensely performed primarily on PPV (poly-p-phenylenevinylene)-based devices, poly-thiophene based devices, and the like. However, these systems have drawbacks in that it is difficult to increase the purity, and the fluorescent quantum yield is essentially low. Therefore, any high performance organic electroluminescent device has not been-produced under the present circumstances. In consideration that polymer materials in a glass state is essentially stable, an excellent organic electroluminescent device can be provided if a high fluorescent quantum yield can be imparted to the polymer materials. As described above, it is known that organic electroluminescent devices through the use of low-molecular materials and through the use of polymers have advantages as well as drawbacks.
Recently, research has also been intensely performed on the improvement of efficiency through the use of triplet excitons (refer to T. Tsutsui et al., Jpn. J. Appl. Phys. Vol. 38, L1502 (1999); and C. Adachi, M. A. Baldo, S. R. Forrest and M. E. Thompson, Appl. Phys. Lett., Vol. 77, 904 (2000), and the like), and it has been made clear that the luminous efficacy has been improved significantly. In addition, reports on host materials used for luminescent layers have been increased. Among them, typical examples of host materials include 4,4′-bis(carbazolyl-9)biphenyl (CBP) represented by the formula described below. However, it has been made clear from the following research that crystallization of CBP has proceeded in the luminescent layer, resulting in shortening of the life of the device (refer to WO01/72927 A1, for example).

In attempting to avoid this problem, researches on carbazole compounds in place of CBP have been described in WO01/72927 A1, Japanese Unexamined Patent Application Publication No. 9-310066, and the like. Furthermore, researches on methods for synthesizing carbazole compounds and electrochemical behaviors of the carbazole compounds have also been performed (refers to M. Park et al., Tetrahedron 54 (1998) 12707 to 12714, W. Laum et al., Journal fuer Praktische Chemie (Leipzig) 317 (6) (1975) 995 to 1004, and the like). However, every compound described above does not have a desired durability and luminous efficiency.